Certain embodiments of the present invention relate to an electron beam tomography (EBT) scanner. More particularly, certain embodiments relate to a method and apparatus for generating an image from a data set collected from a subject beginning at an arbitrary time within a scanning time interval.
EBT scanners are generally described in U.S. Pat. No. 4,352,021 to Boyd, et al. (Sep. 28, 1982), and U.S. Pat. No. 4,521,900 (Jun. 4, 1985), U.S. Pat. No. 4,521,901 (Jun. 4, 1985), U.S. Pat. No. 4,625,150 (Nov. 25, 1986), U.S. Pat. No. 4,644,168 (Feb. 17, 1987), U.S. Pat. No. 5,193,105 (Mar. 9, 1993), U.S. Pat. No. 5,289,519 (Feb. 22, 1994), U.S. Pat. No. 5,719,914 (Feb. 17, 1998) and U.S. Pat. No. 6,208,711 all to Rand, et al., and U.S. Pat. No. 5,406,479 to Harman (Apr. 11, 1995). Applicants refer to and incorporate herein by reference each above listed patent to Rand, et al. and Harman.
As described in the above-referenced Rand et al. patents, an electron beam is produced by an electron gun at the upstream end of an evacuated, generally conical shaped housing chamber. A large negative potential (e.g. 130 kV or 140 kV) on the electron gun cathode accelerates the electron beam downstream along the chamber axis. Further downstream, a beam optical system that includes solenoid, quadrupole, and deflection coils focus and deflect the beam to scan along an X-ray producing target. The final beam spot at the X-ray producing target is smaller than that produced at the electron gun, and must be suitably sharp and free of aberrations so as not to degrade definition in the image rendered by the scanner.
The X-rays produced by the target penetrate a patient or other object and are detected by an array of detectors. The detector array, like the target, is coaxial with and defines a plane orthogonal to the scanner axis of symmetry. The output from the detector array is digitized, stored, and computer processed to produce a reconstructed X-ray video image of a slice of the object, typically an image of a patient's anatomy such as the heart or lungs.
An EBT scanner allows for the collection of many angles of view and scanning of a number of slices in a short time. There is no mechanically moving gantry. Both high resolution and dynamic scanning modes may be provided while eliminating the need for any target or detector motion by replacing conventional X-ray tubes with electron beam technology.
Multiple views may be generated by magnetically steering a focused electron beam along a 210 degree target ring positioned beneath a subject. Opposite the target ring is a stationary detector ring of Cadmium tungstate crystals encompassing a 216 degree arc above the subject. Photodiodes in the detector ring are used for recording transmitted X-ray intensity.
Each scan of a target ring requires 52 milliseconds followed by a 6 millisecond reset time before starting the next scan. The high-speed capability of the EBT scanner offers significant advantage over the gated conventional CT method and images over much of the entire apex-to-base extent of the heart or lungs may be obtained. Since movement by the heart and lungs may degrade image quality, it is extremely important to complete a scan in a limited amount of time. EBT technology allows for the high-speed scanning that is required.
CT scanners that do not use electron beam technology typically scan and collect data over a full 360 degrees. Data is taken continuously during a spiral scan and the data used to generate an image may be selected to begin at any point during the scan. As a result, the image time is independent of when the scan was triggered which is of value, for example, when searching for nodules in the lungs.
For scanners using electron beam technology, it is not possible to collect the data over 360 degrees. A data set is instead collected over, typically, 210 degrees. During scanning, the electron beam crosses a beam stop for typically a 6 millisecond interval between scans. Therefore, data sets are temporally separated from each other by the 6 millisecond gap.
Currently, the temporally separated data sets are each separately reconstructed into an image. As a result, the time for each image slice is determined by the initial conditions at the beginning of the corresponding scan and may not be varied in a continuous manner. As a result, the temporal discontinuities between temporally adjacent scans may cause information to be missed or unwanted artifacts, such as streaks, to appear in an image when trying to reconstruct the image across a temporal boundary.
It is often desirable to construct images starting at arbitrary times within the scanning interval requiring imaging across scan slice (sinogram) temporal boundaries. Therefore, electron beam scanners are at a disadvantage, due to the temporal discontinuities, compared to conventional CT scanners.
For example, in EBT angiography, scans are taken repeatedly at the same z-position through the patient during each cardiac cycle. Also, if the table that the patient is laying on is moved at a constant velocity along the z-axis, then the desire to create an image at an arbitrary time during the scanning time interval is equivalent to creating an image at an arbitrary z-position. CVS (continuous volume scanning) attempts to center the image data about an arbitrary z position when searching for nodules in the lungs.
A need exists to compensate for image artifacts and loss of information due to temporal separation between temporally adjacent sinograms produced by an EBT scanner such that construction of quality images across sinogram temporal boundaries is achieved.